Pretty sure it was the whole anti-vax thing, actually

She'd said some kooky stuff in the past but she'd already retracted pretty much all of it before she even ran and didn't run on a campaign of antivax. Biden had a history of racism and he was heavily involved in pushing the US into Iraq but he still became president.

I trust her retractions about as far as I can throw them, to be honest. And Biden would not have been my choice in that primary either